The systems like the ones you link on here ky*weremiej are very expensive to run, the Tropica option that Sophie linked or the Tetra optimat do use less Co2 as you dose once perhaps twice a day and the trapped Co2 slowly becomes dissolved within the water however it's only ever going to be a low value of dissolved Co2 and probably nowhere near the recommended 20 mg/l , the bubble type gassing disuser uses more gas even to reach the same value mg/l as the gas will escape to the surface on even the smallest produced bubbles. With Yeast type Co2 generators and a little experimentation with mixtures you can get an effectively cheap way to get desired dosing for healthy plant growth.